Electrical contact



J. B. FORD ELECTRICAL CONTACT March 3, 1936.

Filed Feb. 1, 1955 Inventor: John B. For" by W His Attorney.

Patented Mar. 3, 1936 PATENT, OFFICE ELECTRICAL CONTACT John B. Ford,Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application February 1, 1935, Serial No. 4,483

3 Claims.

My invention relates to electrical contacts of the silver contact typeand has for an object the provision of an inexpensive low resistancesilver contact provided with a backing member which may be readilyunited integrally with an iron or steel supporting member. I

Whilesilver has been commonly recognized as a good contact making andbreaking material, its use has been restricted due to the diflicultiesof securing the silver metal to steel supporting members. Heretofore thesilver contacts have been formed as by coining or by the flowing of thesilver metal under pressure into predetermined size, the pre-formedcontact then being applied to the supporting member either by rivetingor brazing. While silver can be welded to iron or steel by exercising agreat deal of care, welding is not practical for quantity production ofswitch members where the silver tips are welded directly to the switchmembers due to the .high percentage of rejects, i. e., switch members towhich the silver contacts have not been welded securely. If the silveris secured to the switch member by brazing a flux must be used and thetemperature controlled very accurately. This method is too costly to besatisfactory. Riveting is unsatisfactory since additional operations arenecessary and under the repeated stress of the opening and closing ofthe contacts the rivets often are loosened sufficiently to interferewith the circuit making and breaking functions. It has been heretoforeproposed to provide silver contacts with backing metals of copper or analloy of copper and zinc, the backing metals being used to decrease thecost of the contact and to afford a surface metal which can be securedto a supporting member by brazing or soldering. As I have said, however,brazing or soldering operations are not desirable in quantity productiondue to the great care demanded to insure a minimum number of rejects.

In carrying out my invention in one form thereof I provide a pre-formedelectrical contact having a body portion formed of silver and a backingmember integrally united to the silver, the backing member being formedof steel. With such a pre-formed contact the steel membar can beresistance welded to a supporting switch member of steel or the likewithout the use of flux or other complicated manufacturing methods. Ivhave found that by integrally uniting the steel backing member and thesilver contact members prior to the mounting of the silver contacts tothe switch members that the pre-formed contacts can be resistance weldedto their supporting members by an automatic welding machine, the unionbetween the steel supporting member and the steel backing member in allcases affording a strong and rigid connection. The result is that thelife of the contacts is lengthened, the resistance is decreased, and thecost of the preformed contact is decreased over a silver contact ofcorresponding size.

In order to assist in the positioning of the preformed contacts withreference to the switch member, the backing member is provided with oneor more positioning projections which are arranged to nest within acentering groove provided in the switch member. Due to the thinning ofthe material inthe switch member by the groove and the relatively smallcross'sectional area of the projections, the maximum heat during theresistance welding is produced at the projections. During the weldingthese projections flow into the groove, the flowing of the metal rapidlyextending to the adjoining surfaces of the switch member and the backingmember to provide an extremely strong union between the two.

For a more complete understanding of my invention reference should nowbe had to the accompanying drawing wherein I have shown in Fig. 1 myinvention applied to a switch member provided with a pair of silvercontacts each having a backing member of steel; Fig. "2 is a perspectiveview of a pre-fornied silver-steel contact and one end of the switchmember shown in Fig. 1 prior to the union of the steel backing memberwith the switch member; Fig. 3 is a sectional elevation of the contactshown in Fig. 2; while Fig. 4 illustrates a modified form of theinvention.

Referring to the drawing, I have shown my invention in one form asapplied to a steel switch member ID provided with silver-steel contactsH and [2 of identical construction. The silver con-' tact ll consists ofa body portion l3 formed of fine silver and a ferrous backing member Hformed of free flowing steel composed mainly of iron.Preferablyrthessteel is heat treated for maximum ductility, i. e., freeflowing. The high ductility is desirable since it facilitates thepreferred method of integrally uniting the silver to the steel byrolling together under pressure silver and steel bars-which have beenheated to a predetermined temperature. After the relatively large barshave been thus integrally united and rolled to a predeterminedthickness, composite members approximately the size of the contacts arestamped from the composite metal.

The finishing operation is performed by placing the composite members indies and applying pressure of a magnitude suflicient to cause the metalsto flow into the dies and form the finished contacts.

Instead of the hot rolling method, the bars or plates may be integrallyunited by other suitable methods well known to those skilled in the artsuch, for example, as by brazing.

The steel backing member ll of the finished contact is provided with aplurality of frustroconical projections it, each of which is located thesame distance from the center of the backing member. These projectionsof gradually increasing cross-sectional area from their extremities totheir base cooperate with a groove !6 formed in the end I! of the switchmember l and serve to position the contact I l on the switch memberpreparatory to the welding operation. The groove l6 shown as circular inform is complementary to the projections; that is to say, it

of the resistance "welding machine are then brought to bear on the outerfaces of the respective contact members to'complete an electricalconnection and to apply a predetermined pressure .on the contacts. Byreason of the groove l6 and the dimensioningof-the projections 15 thewelding current flowing from one electrode to the other tends toconcentrate through the path of lowest resistance provided by theprojections I and the thinned portion of the switch member III at thebottom of the groove. -The result is that the projections II are firstmelted, the metal flowing along the groove and uniting with the meltedwalls of the groove. A strong union of the steel backing member M to theswitch member III is therefore in everycase assured and I have foundthat faulty switch members have been practically eliminated because ofimproper welding of the parts. in fact, after the steel backing member Ihas been welded to the switch member it is practically impossible toseparate them.

As shown in Fig. 3, the thickness of the silver body portion ll ispreferably somewhat greater 10 than the thickness of the backing memberI 4 although it is understood that the relative thicknesses can bevaried as desired. Preferably, the backing member I4 is formed slightlyconcave so that the outer rim Ha overlaps somewhat the face of theswitch member III. This construction is desirable because it reduces thethickness of the resulting switch member and contact assembly.

If desired, the silver body portion it can be 2 formed to overlap theouter edges of the backing member l4, this construction being shown inFig. 4. v

While I have shown a particular embodiment of my invention, it will beunderstood, of course, 2s- -that I do not wish to be limited theretosince many modifications may be made, and I, therefore, contemplate bythe appended claims to cover any such modifications as fall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:--

1. An electrical contact member comprising a body portion formed ofsilver and a backing member formed of a ferrous metal integrally securedto said body portion.

2. An electrical contact member comprising a body portion formed ofsilver, a backing member for said body portion formed of steel, theadjoiningsurfaces of said body portion and said back- 40 ing memberbeing integrally secured together.

